ORGANIC LETTERS
Electrochemical Synthesis and Mechanestic Study of Quinone Imines Exploiting the Dual Character of N,N-Dialkyl-pphenylenediamines
2011 Vol. 13, No. 8 1928–1931
Abbas Maleki and Davood Nematollahi* Faculty of Chemistry, Bu-Ali-Sina University, Hamedan 65178 38683, Iran
[email protected] Received January 26, 2011
ABSTRACT
Two one-pot electrochemical approaches for the synthesis of similar quinone imines via either the nucleophilic character of N,N-dialkyl-pphenylenediamines or electrophilic reactivity of their oxidized forms through Michael-type addition reactions are reported.
Synthesis of molecules with desirable functional groups in the absence of catalysts and drastic conditions continues to be of great interest. In organic synthesis, electrochemical electron-transfer-driven reactions have been widely used for various transformations.1 However, their potential has not yet been fully utilized. Quinone imines and diimines have been of long-standing interest in chemistry.2 Quinone imine dyes are commonly used as redox indicators3,4 as well as solvent polarity indicators.5 The preparation of simple quinone imines by chemical oxidation of aniline derivatives is often complicated by the reactivity of the quinone imine under the reaction conditions. Thus, the electrochemical (1) Yoshida, J.; Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev. 2008, 108, 2265. (2) Swenton, J. S.; Shih, C.; Chen, C.; Chou, C. J. Org. Chem. 1990, 55, 2019. (3) Barra, M.; Croll, L. M.; Tan, A.; Tao, W. Dyes Pigm. 2002, 53, 137. (4) Ottaway, J. M. Indicators; Bishop, E., Ed.; Pergamon Press: Oxford, 1972; Chapter 3A. (5) Reichardt, C. Solvents and solvent effects in organic chemistry; VCH: Weinheim, 1990; Chapter 6. 10.1021/ol103146k r 2011 American Chemical Society Published on Web 03/18/2011
synthesis of stable quinone imines is desirable. In previous studies, we have shown that N,N-dialkyl-p-phenylenediamines and catechols can be oxidized electrochemically to corresponding quinone diimines6 and quinones,7 respectively, that can, further, be attacked as Michael acceptors by a variety of nucleophiles. To the best of our knowledge, designing two synthesis platforms in order to synthesize a common pruduct with a specific functional group via two (6) (a) Nematollahi, D.; Maleki, A. Electrochem. Commun. 2009, 11, 488. (b) Maleki, A.; Nematollahi, D. Electrochem. Commun. 2009, 11, 2261. (7) (a) Nematollahi, D.; Habibi, D.; Rahmati, M.; Rafiee, M. J. Org. Chem. 2004, 69, 2637. (b) Nematollahi, D.; Rafiee, M. Green Chem. 2005, 7, 638. (c) Davarani, S. S. H.; Nematollahi, D.; Shamsipur, M.; Najafi, N. M.; Masoumi, L.; Ramyar, S. J. Org. Chem. 2006, 71, 2139. (d) Nematollahi, D.; Goodarzi, H. J. Org. Chem. 2002, 67, 5036. (e) Nematollahi, D.; Amani, A.; Tammari, E. J. Org. Chem. 2007, 72, 3646. (f) Nematollahi, D.; Workentin, M. S.; Tammari, E. Chem. Commun. 2006, 1631. (g) Nematollahi, D.; Afkhami, A.; Tammari, E.; Shariatmanesh, T.; Hesari, M.; Shojaeifard, M. Chem. Commun. 2007, 162. (h) Nematollahi, D.; Tammari, E. J. Org. Chem. 2005, 70, 7769. (i) Nematollahi, D.; Rafiee, M.; Fotouhi, L. J. Iran. Chem. Soc. 2009, 6, 448.
Figure 1. Cyclic voltammograms of (I,a and II,a) 1 mM 1a; (III,a and IV,a) 1 mM 1b; (I,b) 1 mM 1a in the presence of 1 mM 3; (II,b) 1 mM 1a in the presence of 1 mM 4; (III,b) 1 mM 1b in the presence of 1 mM 3; (IV,b) 1 mM 1b in the presence of 1 mM 4; (I,c) 1 mM 3; and (II,c) 1 mM 4, at glassy carbon electrode, in aqueous solution containing 0.2 M phosphate buffer (pH = 8.0). Scan rate: 20 mV s 1.
different pathways where a reagent with either nucleophilic or electrophilic character is used has not yet been reported. In the current study, we present two one-pot easy electrochemical approaches by which N,N-dialkyl-p-phenylenediamines having nucleophilic or electrophilic reactivities can be successfully applied to the synthesis of similar quinone imines. Either the nucleophilic characteristics of N,N-dialkyl-p-phenylene diamines or electrophilic reactivity of their oxidized forms which subsequently engage in the Michael addition reaction were utilized for the synthesis of similar quinone imines. The following two electrochemical strategies based on the dual character of N,N-dialkyl-p-phenylenediamines, electrochemical oxidation of N,N-diethyl-p-phenylenediamine (1a) and N,N-dimethyl-p-phenylenediamine (1b) in the presence of phenol (3) and 1-naphthol (4) as nucleophiles, were studied. In addition, the electrochemical oxidation of 4-tert-butylcatechol (1c) in the presence of 1a and 1b as nucleophiles was investigated. (8) (a) Seymour, E. H.; Lawrence, N. S.; Compton, R. G. Electroanalysis 2003, 15, 689. (b) Kershaw, J. A.; Nekrassova, O.; Banks, C. E.; Lawrence, N. S.; Compton, R. G. Anal. Bioanal. Chem. 2004, 379, 707. (c) Lawrence, N. S.; Davis, J.; Jiang, L.; Jones, T. G. J.; Davies, S. N.; Compton, R. G. Electroanalysis 2000, 12, 1453. (d) Lawrence, N. S.; Davis, J.; Jiang, L.; Jones, T. G. J.; Davies, S. N.; Compton, R. G. Electroanalysis 2001, 13, 432. (e) Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Anal. Chem. 2003, 75, 2054. (f) White, P. C.; Lawrence, N. S.; Davis, J.; Compton, R. G. Anal. Chim. Acta 2001, 447, 1. Org. Lett., Vol. 13, No. 8, 2011
In the first strategy, the electrochemical oxidation of 1a was studied in the presence of 3. Cyclic voltammetry of 1 mM of 1a shows one anodic peak (A1) and its cathodic peak (C1), which correspond to the transformation of 1a to quinonediimine 2a and vice versa, through a quasi-reversible twoelectron process (Figure 1I,a).6,8 Under these experimental conditions, a peak current ratio (IpC1/IpA1) that is close to 1 can be considered as a criterion for the stability of 2a. In the presence of 3, the first cycle of the voltammogram of 1a shows a decrease in the cathodic peak C1 and the appearance of a new cathodic peak (C0) in the more negative potentials (Figure 1I,b). In the second cycle, a new anodic peak (A0), which is the counterpart of C0, appears with an Ep value of 0.05 V versus SCE. This new peak is related to the electrooxidation of intermediate 6a. Furthermore, with increasing the potential sweep rate and the decrease in the peak height of C0, the peak current ratio (IpC1/IpA1) increases. An increase in the ratio of (IpC1/IpA1) and (IpC1/IpC0) with increasing the scan rate for a mixture of 1a and 3 confirms the reactivity of 2a toward 3 (see Supporting Information, Figure S1). On the other hand, the current function for peak A1 (IpA1/v1/2) decreases with increasing the scan rate and such a behavior is an indication of an ECE mechanism.9 Monitoring the electrolysis progress by cyclic voltammetry synchronously during controlled-potential coulometry in an aqueous solution containing 1a and 3 at 0.15 V versus SCE shows that as the coulometry progresses, the IpA1 and IpC1 decrease (See Supporting Information, Figure S2). These peaks (A1 and C1) disappear when the charge consumption becomes about 4e per molecule of 1a. Furthermore, time-dependent absorption spectra of the mixture of 1a and 3 were measured during a controlled-potential coulometry experiment (see Supporting Information, Figure S3). As the coulometry experiment progresses, absorption peaks with λmax at 275 and 670 nm that belong to the blue color of the product 7a appear and their heights gradually increase. The diagnostic criteria of cyclic voltammetry, the consumption of four electrons per molecule of 1a, and the spectroscopic data of the final product support the quinone-imine structure of 7a. According to our results, the Michael addition reaction of the anion phenolate 3 to quinone-diimine 2a is faster than other secondary reactions, leading to the intermediate 6a. Because the oxidation of intermediate 6a is easier than the oxidation of 1a, following the chemical reaction the apparent number of electrons transferred increases from 2 to 4 electrons and quinone-imine 7a is synthesized. The electrochemical oxidation of 1a in the presence of 1-naphthol (4) proceeds in a similar way to that of 3 (see Supporting Information, Scheme S1) and produces quinone-imine 9a as the final product (see Scheme 1, inset). However, its cyclic voltammogram shows an intense increase in the current for peak A1 as well as a complete disappearance of peak C1 even with high scan rates (Figure 1II,b). This might be related to the adsorption of 4 on the surface of the electrode. Under these conditions, the surface concentration of 4 is high and remains (9) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; Wiley: New York, 2001; pp 512 515. 1929
Scheme 1. Strategy 1 for the Synthesis of Quinone Imine
Scheme 2. Electrochemical Synthesis of 11b by Strategy 1
essentially unchanged during the voltammetric experiment. However, 2a is gradually generated at the electrode surface and reacts with the large amount of 4. In this case, the rate of reaction of 4 with 2a increases and subsequent oxidation leads to a fast growth in the current of peak A1. The rapid reaction of generated 2a with 4 causes the surface concentration of 2a to become effectively zero. Such behavior is a general treatment of the ECE system with pure kinetic conditions.9 To follow this strategy for the electrochemical synthesis of quinone imines, electrochemical oxidation of 1b in the presence of 3 and 4 was studied under the same conditions. The electrochemical oxidation of 1b in the presence of 3 proceeds similarly to that of 1a (Scheme 1); however, the electrochemical oxidation of 1b in the presence of 4 proceeds differently. Cyclic voltammograms of 1b in the presence of 4 have been shown in Figure 1IV,b. Controlled-potential coulometry of 1b in an aqueous solution containing 0.2 M phosphate buffer (pH = 8.0) at 0.15 V versus the SCE yields an napp value of about 3. The diagnostic criteria of cyclic voltammetry, the consumption of three electrons per molecule of 1b, and the spectroscopic data of the isolated product indicate that the reaction mechanism of the electrooxidation of 1b in the presence of 4 is ECECE (Scheme 2). According to our results, it seems that the Michael-type addition reaction of 4 to quinone-diimine 2b leads to the intermediate 8b. Another oxidation process converts 8b to quinone-diimine 9b, and next Michael addition of 1b to 9b transforms 9b to intermediate 10b. Because of the presence of an electrondonating group, the oxidation of this compound (10b) is easier than the oxidation of 1b. Further oxidation converts intermediate 10b to the final product 11b. Based on Scheme 2, in Figure 1IV,b, the anodic peaks of A1 and A2 pertain to the oxidation of 1b and 8b to quinonediimines 2b and 9b, respectively. Also the anodic peak A0 and its counterpart (C0) correspond to the oxidation of 10b to 11b and vice versa. Another less probable pathway for the formation of 11b is proposed in Supporting Information, Scheme S2. In the second strategy for the electrochemical synthesis of quinone imines, 1a and 1b act as nucleophiles. Using cyclic voltammetry, the electrochemical oxidation of
4-tert-butylcatechol (1c) was studied in the presence of 1a and 1b as nucleophiles in an aqueous solution containing 0.2 M phosphate buffer (pH = 7.5). Cyclic voltammetry of 1c shows one anodic (A1) and the corresponding cathodic peak (C1), which correspond, through a quasi-reversible two-electron process, to the transformation of 1c to related o-benzoquinone 2c and vice versa (Figure 2, curve a).7 Also one anodic (A2) and the corresponding cathodic peak (C2) were observed in the cyclic voltammograms of 1b alone (Figure 2, curve b). Figure 2 (curve c) shows the cycle voltammogram of 1c in the presence of 1b. Under these conditions, IpA1 increases and its cathodic counterpart (C1) decreases. Also the voltammogram exhibits a new cathodic peak C0 at more negative potentials. In the second cycle (curve d), a new peak (A0) appears with an Ep value of 0.14 V versus SCE. This new peak is related to the oxidation of intermediate 13b. Furthermore, the height of the peak C1 increases with increasing potential sweep rate (see Supporting Information, Figure S4). This indicates the reactivity of electrochemically generated o-benzoquinone 2c toward 1b. These observations are indicative of an ECE mechanism9 and allow us to propose a mechanism for the electrooxidation of 1c in the presence of 1a,b (Scheme 3). Accordingly, the 1,4-addition (Michael) reaction of 1a,b to the o-benzoquinone 2c leads to the intermediate 13a,b. The oxidation of 13a,b is easier than the oxidation of 1c due to the presence of an amine group as an electron-donating group. Therefore, 1c in the presence of 1a,b after consumption of four electrons converts to quinone-imine 14a,b. In the case where two substrates are susceptible to oxidation and their oxidation potentials are close to each other, the
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Scheme 3. Strategy 2 for the Synthesis of Quinone Imine
Figure 2. Cyclic voltammogram of 1 mM (a) 1c, (b) 1b, (c) 1c in the presence of 1b, and (d) initial part of second cycle of cyclic voltammogram of 1c in the presence of 1b, at glassy carbon electrode, in aqueous solution containing 0.2 M phosphate buffer (pH = 7.5). Scan rate: 50 mV s 1; t = 25 ( 1 °C.
choice of an appropriate potential is often crucial for the success of an electrode reaction. In this case, for the selective oxidation of 1c, a controlled-potential method was used. In contrast to one-pot chemical synthesis where there is no control on the oxidation of a single component, controlled-potential electrolysis serves as a powerful method for the synthesis of a desirable product via selective oxidation of a target reagent. As expected, at the small applied potential of 0.05 V, the selective electrochemical oxidation of 1c proceeds preferentially, thereby providing the desired quinone imine 14a,b. From the point of view of green chemistry, the use of an electrosynthesis method has some important advantages. Clean synthesis, the use of electricity instead of chemical reagents, and the achievement of high atom economy via a
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one-step process conducted under ambient conditions are attractive features with respect to green chemistry ideas. Although one-pot reactions are performed potentiostatically on a mmol scale in divided cells, there is little difficulty in producing larger quantities by using larger cells. Information for the preparative electrolysis including the potential, electricity, temperature, and yields is described in the Supporting Information. Acknowledgment. We thank Prof. Wolfgang Schuhmann at the Ruhr-Universit€at Bochum-Germany for the high resolusion mass spectra. The authors acknowledge the Bu-Ali Sina University Research Council and the Center of Excellence in Development of Chemical Methods (CEDCM) for supporting this work. Supporting Information Available. Experimental procedures, some electrochemical investigations, and full spectroscopic data for all compounds. This material is available free of charge via the Internet at http://pubs. acs.org.
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